1. Field of the Invention
This invention generally relates to immersion heaters and more specifically to immersion heaters for converting liquids into gas as, for example, by generating steam from water for use in a wide variety of applications.
2. Description of Related Art
Humidifiers, distilling apparatus, water kettles and coffee brewing machines are some devices that use immersion heaters to heat a liquid or to convert a liquid, such as water, into vapor or steam by boiling water. In humidifiers steam mixes with air and disperses throughout a room to increase relative humidity. In a distilling apparatus a condenser receives the steam to produce distilled water. In water kettles the immersion heaters boil or heat cold water to a desired temperature. In many coffee makers, immersion heaters boil water to enable the water to percolate through coffee grounds to produce hot coffee.
Many embodiments immersion heater types exist. However, characteristically over time deposits or residues from the water adhere to the surface of such immersion heaters. These deposits form an insulating layer that impedes the transfer of heat to the water thereby reducing the effectiveness of the immersion heater. Cleaning of these deposits or residues becomes very difficult.
U.S. Pat. No. 5,111,529 (1992) to Glucksman, that is assigned to the same assignee as this invention, discloses one embodiment of an immersion heater used with a portable air humidifier. A housing contains an evaporation chamber in the shape of an inverted cup. The chamber has a steam outlet in its top and an electric heating element positioned a short distance above an open bottom. In this particular humidifier a blower delivers cool air through a duct to communicate with the evaporation chamber through its steam outlet to blow humidified air back into the room. Water from a main compartment or tank transfers to a compartment in a removable tray that allows ready access to the compartments for cleaning.
U.S. Pat. No. 4,818,344 (1989) to Glucksman, that is assigned to the same assignee as this invention, discloses a water distilling apparatus with a vaporization chamber containing an electric coil heating element, a fin tube condenser coil and an electric fan for cooling the surfaces of the condenser coils. An electric coil heating element vaporizes water and the resulting steam travels through the condenser tubing where it reconverts to pure water for delivery to a beaker through a water outlet. It is possible to remove various portions of this device for cleaning sediments and residues.
U.S. Pat. No. 5,835,680 (1998) to Glucksman et al., that is assigned to the same assignee as this invention, discloses an immersible heater with an annular collar that spaces a heating element above a bottom wall of the structure. The collar acts as an insulator thereby to limit the heat transferred to the bottom wall that is typically formed of a plastic material. In addition, a thermostat monitors the temperature of the heater. When the water is nearly evaporated, the thermostat or a backup thermofuse turns off the heater. This occurs while water still surrounds at least the collar and therefore prevents damage from overheating.
In another humidifier sold by Slant Fin, a stainless steel cup holds water to be boiled. A heating coil attaches to the exterior of the cup and heats the water in the cup to the boiling point. It is easier to clean the interior of this cup as compared to prior cups. However, like the other devices described above, during cleaning it is necessary to handle an entire base unit. However, such base units generally contain electrical components. Such base units generally can not be immersed in water or be cleaned in a dishwasher.
Each of the foregoing patents discloses a heating element that, like other heating elements of the prior art, becomes difficult to clean. As known, water available through various municipal and other supplies contains minerals that do not evaporate along with the water. Certain of these minerals, especially lime, adhere to the heat transfer area, namely to the outer surface of the heating element. As this residue accumulates, the heat transfer rate to the water decreases with two adverse effects. First, the process by which the boiling occurs becomes less efficient. Secondly, the heating coils will operate at a higher temperature. Moreover, as also known, the residue becomes more difficult to remove as it accumulates over time. Although a stainless steel cup reduces any cleaning effort, a requirement to carry an entire humidifier base to a sink for cleaning remains.